Newer designs and manufacturing techniques have driven electronic components to small dimensions and miniaturized many communication devices and systems. Unfortunately, antennas have not been reduced in size at a comparative level and often are one of the larger components used in a smaller communications device. It becomes increasingly important in communication applications to reduce not only antenna size, but also to design and manufacture a scalable size antenna having sufficient gain on the frequency needed. Accurate antenna tuning is important for small narrowband antennas.
In current, everyday communications devices, many different types of patch antennas, loaded whips, copper springs (coils and pancakes) and dipoles are used in a variety of different ways. These antennas, however, are sometimes large and impractical for a specific application. Antennas having diverging electric currents may be called dipoles, those having curling electric currents may be loops, and dipole-loop hybrids may comprise the helix and spiral. While dipole antennas can be thin linear or “1 dimensional” in shape, loop antennas are at least 2 dimensional. Loop antennas can be a good fit for planar requirements.
Antennas can of course assume many geometric shapes. The Euclidian geometries are sometimes preferential for antennas as they convey optimizations known through the ages. For instance, line shaped dipoles may have the shortest distance between two points, and circular loop antennas may have the most enclosed area for the least circumference. So, both line and circle shapes may minimize antenna conductor length to increase radiation efficiency. Yet line and circle shaped antennas may not meet all needs, such as operation at small physical size relative wavelength and a self loading antenna structure may be needed, such as a helix or spiral antenna.
Simple flat or patch antennas can be manufactured as printed circuit boards (PCBs) at low costs and have been developed as antennas for the mobile communication field. The microstrip patch antenna is configured, for example, by disposing a patch conductor cut to a predetermined size over a conductive “ground” plate through a dielectric material. An elegant compound design results: one or more patch edges may radiate as slot antennas, a transmission line impedance matching transformer is obtained, unidirection radiation can be provided, and patch sizing allows synthesis of radiation pattern shapes. The patch may even be excited for linear, circular, and dual polarizations. Patch efficiency may exceed 90%. For comparision, parabolic reflectors may operate at only 50 to 80 percent efficiency, due to factors of feed spillover, non uniform aperture illumination, and surface tolerances. In fact, few or no antennas exceed patch antennas in realized gain for area. Patch arrays may exceed Gr>10 log10 [(0.9)4πa/λ2], where Gr is realized gain in dBi, a is the area of the patches in square meters, and λ is the free space wavelength in meters.
However, microstrip patch antennas typically are efficient only in a narrow frequency band. They are poorly shaped for wave expansion, such that microstrip antenna bandwidth is proportional to antenna thickness. Bandwidth can even approach zero with vanishing thickness (for example, see Munson, page 7-8 “Antenna Engineering Handbook”, 2nd ed., H. Jasik ed.). Limitations of narrow instantaneous radiation bandwidth are potentiated by any variation in PWB substrate dielectric constant; tuning drift may cause the high gain may be unavailable on the frequency needed. This can be problematic when high dielectric constant substrates are used: the miniaturized patch has less fixed tuned bandwidth to mitigate tuning errors, yet high dielectric constant materials typically have wider dielectric constant variations. The typical microstrip patch antenna may not support the whole 1500-1700 MHz mobile satcom band, for example. It also includes sensitive tuning tolerances and production frequency trimming is upwards only (e.g. via patch ablation). Patch resonant frequency is inversely proportional to the square root of substrate dielectric constant (f˜1/√∈r).
U.S. Pat. No. 6,501,427 to Lilly et al. entitled “Tunable Patch Antenna” is directed to a patch antenna including a segmented patch and reed like MEMS switches on a substrate. Segments of the structure can be switched to reconfigure the antenna, providing a broad tunable bandwidth. Instantaneous bandwidth may be unaffected however.
U.S. Pat. No. 7,126,538 to Sampo entitled “Microstrip Antenna” is directed to a microstrip antenna with a dielectric member disposed on a grounded conductive plate. A patch antenna element is disposed on the dielectric member.
U.S. Pat. No. 7,495,627 to Parsche entitled “Broadband Planar Dipole Antenna Structure And Associated Methods” describes a planar dipole-circular microstrip patch antenna with increased instantaneous gain bandwidth by polynomial tuning.
U.S. Pat. No. 7,432,862 to Heyde is directed to a broadband patch antenna including a planar metallic patch sheet that is provided with right-angled edges. U.S. Pat. No. 6,606,061 to Wong et al. is directed to a broadband circularly polarized patch antenna including an L-shaped ground plane consisting of a vertical ground plane and a horizontal ground plane, a radiating metal patch, a probe feed placed coplanarly with the radiating metal patch and connected to the radiating metal patch through the vertical ground plane, and a substrate between the radiating metal patch and the horizontal ground plane.
There may be a desire for a planar patch antenna that may be flexible and/or scalable as to frequency, and provide adequate gain and wide bandwidth.